Lack of a clear understanding of which CD8+ T cell responses and functions mediate control of HIV is substantially hampering development of an HIV vaccine. A major factor limiting CD8 T cell responses is the propensity of HIV to mutate and escape from these responses. However, recent work has begun to illustrate the predictability by which HIV mutates in response to host immune selection pressures and the impact that some CTL escape mutations have upon HIV replication. These studies reveal definable limitations to HIV sequence evolution and illustrate that the impact of some escape mutations on viral replication may be an important contributor to viral control by CD8 T cell responses. Aim 1 will utilize full length viral sequencing to comprehensively identify HLA-associated escape mutations across the HIV proteome. CTL escape mutations will then be examined for their impact on viral replication to enable determining whether HLA alleles associated with slower disease progression restrict CD8 T cell responses from which HIV is unable to effectively evade. Emerging evidence also indicates the ability of the immune system to mount de novo CD8 T cell responses against the variant form of some CTL epitopes following viral escape. To date such responses have been limited to immunodominant responses restricted by `protective'HLA alleles such as HLA-B57, B27, and A11. Coincident with these responses secondary viral escape mutations have also been observed which exhibit a greater impact on viral replication than primary escape mutations. It is unclear, however, the extent to which various CD8 responses are capable of this plasticity and may drive more deleterious secondary escape mutations. Aim 2 will identify the extent to which CD8 T cell responses are capable of mounting variant- specific responses and the impact of secondary escape mutations on viral replication to determine whether such responses are specifically associated with beneficial HLA alleles or protective CD8 T cell responses. Finally, HIV superinfection has been associated with overcoming pre-existing immune control and rapid disease progression, likely due to the substantial sequence diversity between different strains. Thus, studies of superinfection provide a unique opportunity to determine whether loss of immune control is associated with evasion of particular CD8 T cell responses that may disproportionately contribute to initial viral control. Our recent data indicates that following superinfection recombination between strains in critical regions containing targeted CD8 epitopes results in rapid viral escape from these responses, highlighting critical CD8 responses that may have facilitate initial viral control and a novel mechanism of viral escpe. Aim 3 will explore whether recombination commonly occurs within dominant CD8 epitopes following superinfection and whether changes in viral replicative capacity or loss of key CD8 T cell responses correlates with the inability to control a second strain of HIV. Therefore, we will utilize full-length viral genome sequencing, viral replication assays, and cellular assays in these unique settings to identify CD8 T cell responses that may be critical to the control of HIV.